The Acid Test: Armor-Covered Plankton Adapt to Warming World

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Tiny armor-covered creatures that float along with the ocean's
currents may adapt and survive, if badly, as their watery world
warms and becomes more acidic, a new study finds.

Even so, the plankton may become flimsier and could turn into
more of a "french fry" than a nutritious snack for its consumers.

As more carbon dioxide, a greenhouse gas, gets pumped into the
atmosphere, and ultimately dissolves in the oceans, the
seas are becoming more acidic. How this will impact life in
the oceans is not known, though various studies have undertaken
the challenge to find out.

In the new study, a trio of scientists at the Helmholtz Center
for Oceanographic Research in Kiel, Germany, bred a variety of
phytoplankton, called Emiliania huxleyi, to tolerate
higher levels of carbon dioxide dissolved in the water.

They focused on these creatures for two reasons: Like other
phytoplankton, E. huxleyi forms the bases of many of
the ocean's food chains. In addition, this creature is a
coccolithophore, which builds its shell of calcium carbonate.
That shell-building can be affected by the acidity of the oceans,
with
more acidic oceans holding less of their shell material.

Ocean acid test

The pH of the oceans, a measure of acidity whereby lower numbers
are more acidic, has changed from about 8.25 in the mid-18th
century to 8.14 in 2004.

To find out how this change and future changes might impact the
armored
plankton, researchers Kai Lohbeck, Ulf Riebesell and Thorsten
Reusch took the plankton they had bred in the lab and exposed it
to concentrations of carbon dioxide up to four times that in the
atmosphere. They found that it can adapt, and even maintain its
shell-building, though it doesn't exactly thrive. "They do
less badly," Reusch said.

Their ability to adapt and survive in the "harsh" environment
took less than a year (though for plankton, the time frame spans
about 500 generations). Reusch said it's the first time anyone
has studied the evolution of plankton over so many generations.
[ The
Harshest Environments on Earth ]

Reusch noted that the team used plankton with the same genetic
makeup, so whatever changes occurred were at the level of gene
expression. The particular genes that are involved will be the
subject of future experiments, he said.

The team found that under higher carbon-dioxide levels, the
plankton grew faster (and got bigger overall), but they didn't
build shells as quickly. Compared with earlier generations, the
adapted plankton did increase their rate of shell growth, but
that never reached levels found under normal-CO2 conditions.
Essentially, the amount of shell substance per dry weight of
plankton decreased.

Ocean junk food?

Just because
the plankton evolve to tolerate acidic oceans doesn't mean
that the food chains won't be affected. Reusch noted that
altering the water chemistry can also affect how nutritious the
plankton are for the other creatures that eat them, because it
affects their metabolism. "They become like french fries," he
said. "The carbon-nitrogen balance becomes worse," which affects
nutrients needed by those that feast on them, such as zooplankton
— tiny jellyfish, copepods and shrimp.

The finding also has implications for
Earth's storage of carbon. To make their shells, plankton
take two bicarbonate ions from the water and make it into calcium
carbonate and water, releasing an extra molecule of carbon
dioxide (CO2). That means the faster they grow, the more CO2 gets
pumped into the air. But the situation is more complicated,
because the plankton sequester some carbon when they die, taking
it with them to the bottom of the ocean. Plankton also
photosynthesize, like other plants, so they release oxygen as
well as removing CO2 from the air.

An expert on plankton, Larry Brand, a professor of marine biology
and fisheries at the University of Miami in Florida, noted that
one of the big effects of acidifying the oceans is altering the
mix of creatures that live in it. If the Emiliania
plankton doesn't do as well, other plants and animals will take
its place. That could alter fisheries, for example. "Usually when
there's a radical change in the mix of organisms, it doesn't work
out too well for humans," he said.

They published their results in the journal Nature Geoscience
today (April 8).